The invention relates to a process for the decaffeination of green coffee with carbon dioxide in the supercritical state at a pressure of 200 to 300 bar and a programmed stepwise temperature of 60.degree. C. for 4 hours, 70.degree. C. for the second 4 hours and 85.degree. C. for the final 4 hours of the extraction period.
To avoid the known disadvantages of organic solvents such as trichloroethylene or methylchloride, it is within the skill of the art to extract the caffeine from the raw coffee with supercritical gases, especially carbon dioxide. This method has been selected because carbon dioxide, is a naturally occurring product that is not hazardous to the human organism. Apart therefrom, no undesired residues remain in the decaffeinated coffee beans.
More specifically, a process is known for preparing caffeine-free coffee extract by separating the aroma constituents, extracting the caffeine and rearomatizing the decaffeinated product, wherein, first, the coffee oil containing the aroma constituents is removed from the roasted coffee with a dry, supercritical fluid, then the material to be extracted is moistened with water and the caffeine removed from the moistened material to be extracted with a moist, supercritical carbon dioxide, after which the material to be extracted is processed further in a manner know from the prior art. The fluid charged with the caffeine is segregated in a special container by changing the pressure and/or temperature of the stream of carbon dioxide. The dry, supercritical fluid used for removing the coffee oil from the raw coffee and the moist, supercritical carbon dioxide used for the subsequent extraction of the caffeine are above the critical temperature and the critical pressure. For carbon dioxide, the supercritical state means a temperature above 31.3.degree. C. and above approximately 80 at. To protect the aroma and taste it is expressly recommended that both the coffee oil and the caffeine be extracted at 40.degree.-50.degree. C., the pressure preferably being above 180 at., but for practical and economic resons not above 350 at (Austrian Patent No. 319,722).
This prior art process suffers particularly from the disadvantage that relatively large quantities of carbon dioxide, have to be employed to extract the coffee oil as well as the caffeine, and that the decaffeination is relatively time-consuming, because it is performed at a low rate of decaffeination, added to which is the separate processing step of moistening the coffee with water after the coffee oil has been extracted.
It is likewise within the skill of the art to decaffeinate raw coffee in a heated pressure vessel subjecting the supercritical carbon dioxide at 70.degree. C. to a pressure of around 160 at. First, the carbon dioxide moves upward through the water, so that it is charged with water (Austrian Patent No. 190,982). Then, the moist carbon dioxide spreads through the coffee, yielding a portion of its water to the coffee beans and being concurrently charged with caffeine. The carbon dioxide charged with caffeine is passed from the pressure vessel via a heat exchanger to another pressure vessel which is charged with activated carbon. The second pressure vessel is heated to a temperature of around 25.degree. C. The carbon dioxide charged with caffeine is then cooled by the heat exchanger to around 25.degree. C., at which temperature the carbon dioxide becomes liquid. Simultaneously, the water is removed. Therefore, a stream of some water with a considerable amount of liquid carbon dioxide, and caffeine dissolved therein enters the second pressure vessel. When this stream passes through the pressure separator, the caffeine is retained on the activated carbon. The liquid carbon dioxide and the water are returned to the first pressure vessel via the heat exchanger attaining once again the supercritical conditions.
Instead of an adsorption of the caffeine on activated carbon in the pressure separator, another version is known whereby the caffeine is also removed from the carbon dioxide by allowing it to expand below the critical pressure.
With the prior art process described above the caffeine can be extracted from the raw coffee, but large quantities of carbon dioxide are required, and in addition, a correspondingly long extraction time is required. Indeed, in this connection a selected value temperature in the first pressure vessel should be set in the range from 40.degree. to 80.degree. C. and the selected pressure should be set in the range from 120 to 180 at. This leads to operation in isothermal and isobaric conditions. However, at sustained higher temperatures above 70.degree. C. one has to be prepared for losses in the quality of the decaffeinated coffee.
Large quanities of carbon dioxide are necessary in the above-mentioned temperature and pressure range for the production of 97% caffeine free green coffee. At least 50 kg of carbon dioxide are needed per hour per kg of coffee per hour. For extended extraction times this requires large equipment dimensions as well as a high consumption of power for pumps, heat exchangers and other system components. The large quantities of carbon dioxide which, in accordance with the prior art process, are to be circulated in the system make it particularly difficult to remove the caffeine from the charge carbon dioxide, because the caffeine concentration is correspondingly low. Therefore, in order to remove the caffeine one needs an expensive activated-carbon bed on which caffeine can be absorbed. The caffeine is then practically lost because of the prohibitive cost of extracting it from activated carbon.
It is also known that green coffee with its natural water content can be used for the conventional decaffeination with water and chlorinated solvents (Process of General Foods; Dowe Egberts). However, these prior art processes have never been practiced industrially, suggesting the processes have problems.